(1) Field of the Invention
The present invention relates to a magnetic pattern forming method and apparatus for use in a magnetic recording apparatus (magnetic disk), and more particularly to a magnetic pattern forming method and apparatus, magnetic disk and magnetic recording apparatus suitable for recording/reproduction (readout) by a flying/contact head.
(2) Description of the Related Art
Magnetic recording apparatus represented by a magnetic disk unit (hard disk drive) have come into widespread use as an external storage of information processing equipment such as computers, and in the recent years, they are being used as a moving-picture recording apparatus or a recording apparatus for a set-top box.
The magnetic disk unit (hard disk drive) is usually made up of a shaft for fixing one magnetic disk or a plurality of magnetic disks in a skewering manner, a motor for turning the magnetic disk joined through a bearing to the shaft, a magnetic head to be used for recording and/or reproduction, an arm for holding the head, and an actuator capable of shifting the head through the head arm to an arbitrary position above a magnetic recording medium, with the recording/reproducing head being moved at a constant flying quantity above the magnetic recording medium.
Furthermore, in addition to the flying head, there has been proposed the employment of a contact head which contributes to shortening the distance with respect to a medium.
A magnetic recording medium to be mounted on the magnetic disk unit is produced by forming an NiP layer on a surface of a substrate made usually from an aluminum alloy or the like and then forming successively a metal under layer, a magnetic layer (information recording layer), a protective layer, a lubricant layer and other layers thereon after undergoing needed smoothing treatment, texturing treatment and others. Alternatively, it is made by forming successively a metal under layer, a magnetic layer (information recording layer), a protective layer, a lubricant layer and others on a surface of a substrate made of a glass or the like. Among the magnetic recording media, there are a longitudinal magnetic recording medium and a perpendicular magnetic recording medium. In general, longitudinal recording is made on the longitudinal magnetic recording medium.
The protective layer on the magnetic layer prevents the magnetic layer from being damaged due to collision of the flying magnetic head or sliding contact of the contact head, and the lubricant layer provides lubricity between the magnetic head and the medium. This construction enables recording/reproduction by a flying/contact magnetic head. Since the employment of a flying/contact head shortens the distance between the magnetic layer and the head, the considerably higher density information recording becomes feasible, as compared with an optical disk, magneto optical disk or the like using a different type of head.
The magnetic recording medium shows a yearly increase in density, and for realizing this, there have been known a variety of techniques. For example, the approaches involve reducing the flying quantity of a magnetic head, employing a GMR head as a magnetic head, using a high-corecivity magnetic material for a recording layer of a magnetic disk, or reducing the space between information recording tracks on a magnetic disk. For example, for 100 Gbit/inch2, it is said that the track density is necessary to be equal to or more than 100 ktpi.
Each track has a control magnetic pattern formed for control of the magnetic head, for example, a signal for position control of the magnetic head or a signal for synchronous control. An increase in the number of tracks by a reduction of the space of the information recording tracks requires that signals (which sometimes will be referred to hereinafter as “servo signals”) for position control of data recording/reproducing head are accordingly located radially and densely, that is, the signals are more placed, for more precise control.
In addition, there is an increasing requirement for enlarging a data recording area by reducing an area other than for data recording, that is, an area for servo signals and a gap section between the servo area and the data recording area, thus enhancing the data recording capacity. This requires the enhancement in the output of the servo signal or the accuracy of the synchronization signal.
A conventional method used widely for production involves making a hole in the vicinity of a head actuator of a drive (magnetic recording apparatus) to insert a pin with an encoder thereinto for establishing the engagement with the actuator through the pin, thereby moving a head up to a correct position for recording a servo signal. However, since the center of gravity of the positioning mechanism and the center of gravity of the actuator exist at different positions, difficulty is encountered in executing track position control with high accuracy and recording a servo signal with a high precision.
In addition, there has been proposed a technique for formation of irregular servo signals in which a laser beam is placed onto a magnetic disk to deform a disk surface locally for formation of physical irregularities. There are problems which arises with this technique, however, in that an unstable state of a flying head due to the irregularities exerts adverse influence on recording/reproduction, cost rises because there is a need to use a high-power laser beam for the formation of the irregularities, and the one-by-one formation of the irregularities is time-consuming.
New servo signal formation methods have been proposed with a view to eliminating these problems.
As one example, there has been known a method in which a servo pattern is formed on a master disk with a high-corecivity magnetic layer and this master disk is brought closely into contact with a magnetic recording medium so that the magnetic pattern is printed thereon by means of an auxiliary magnetic field from the external (see U.S. Pat. No. 5,991,104).
Another example is that, in a state where a medium is previously magnetized in one direction, a soft magnetic layer with a high permeability but a low corecivity is placed on a master disk in the form of a pattern and the master disk is brought closely into contact with the medium while an external magnetic field is brought to strike thereon. In this example, the soft magnetic layer serves as a shield, and the magnetic pattern is printed in the non-shielded area (see U.S. Pat. No. 3,869,711, EP915456, Digest of InterMag 2000, GP-06).
This technique features the employment of a master disk and the formation of a magnetic pattern on a medium by a strong magnetic field.
In general, since the strength or magnitude of a magnetic field depends upon distance, in recording a magnetic pattern through the use of a magnetic field, the pattern boundary tends to be unclear due to stray magnetic field. In order to minimize the stray magnetic field, it is essential to make the master disk and the medium come closely into contact with each other. Moreover, it is necessary that, as the pattern becomes finer, they are accordingly brought completely into contact with each other without defining a gap therebetween, and usually, they are pressed against each other through the use of vacuum adhesion or the like.
In addition, the magnetic field to be used for print increases as the corecivity of a medium becomes higher, and the stray magnetic field increases consequently; therefore, there is a need to achieve the contact therebetween more completely.
Thus, although the above-mentioned technique is relatively applicable to a flexible floppy disk easy to press or a low-corecivity magnetic disk which does not require too great contact, considerable difficulty is experienced in applying it to a high-density recording magnetic disk using a hard substrate, whose corecivity exceeds 3000 Oe.
That is, in the case of the magnetic disk using a hard substrate, there is a possibility that microscopic dirt particles or the like get between the master disk and the medium at the contact to produce defects on the medium or damage the high-priced master disk. In particular, for a glass substrate, the contact therebetween does not reach satisfaction because the dirt particles get therebetween, which makes the magnetic print impossible or causes cracks on the magnetic recording medium.
Furthermore, with the technique disclosed in U.S. Pat. No. 3,869,711, although a pattern formed obliquely with respect to a track direction of a disk is recordable, the recording is limited to the formation of only a pattern with a low signal strength. For a magnetic recording medium with a high corecivity exceeding 2000 to 2500 Oe, in order to secure a magnetic field strength for print, a soft magnetic material such as a permalloy or cendust, which has a high saturated magnetic flux density, is used unavoidably as a ferromagnetic material (shield material) for a pattern of a master disk.
However, in the case of the oblique pattern, a magnetic field for the reverse magnetization becomes perpendicular to a gap defined by the ferromagnetic layer of the master disk, which makes it difficult to tilt the magnetization in a desired direction. In consequence, a part of the magnetic field escapes to the ferromagnetic layer to make it difficult to expose a desired portion to a sufficient magnetic field at magnetic print so that a sufficient reverse magnetization pattern is difficult to form and a high signal strength is hard to obtain. In the case of such an oblique magnetic pattern, its reproduction output drops significantly beyond the azimuth loss with respect to a pattern perpendicular to a track.
On the other hand, a technique disclosed in Japanese Patent Application Nos. 2000-134608 and 2000-134611 relates to the formation of a magnetic pattern on a magnetic recording medium by means of a combination of local heating and exposure to an external magnetic field. For example, in a state where a medium is previously magnetized in one direction, an energy beam or like is applied through a patterned mask thereonto for the local heating and an external magnetic field is brought to strike thereon while the corecivity of the heated area is lowered, thus making recording on the heated area by the external magnetic field for formation of a magnetic pattern.
According to this technique, since the external magnetic field is brought to strike on a medium while the corecivity is lowered by heating, there is no need for the external magnetic field to be higher than the corecivity of the medium, and the recording becomes feasible with a weak magnetic field. Moreover, since the area subjected to recording is limited to the heating area while the recording in other than the heating area cannot be made even if a magnetic field is brought to strike thereon, the recording of a clear magnetic pattern is possible without bringing a mask or the like into contact with the medium. This can prevent the medium or the mask from being damaged due to the press, thus suppressing the defects appearing on the medium.
In addition, this technique enables the formation of a satisfactory oblique magnetic pattern. This is because there is no need to shield the external magnetic field with a soft magnetic material of the master disk unlike the conventional art.
Thus, the magnetic pattern forming technique disclosed in Japanese Patent Application Nos. 2000-134608 and 2000-134611 is a superior technique which can efficiently form various minute magnetic patterns with high accuracy and suppress the defects of a medium without damaging the medium or a mask.
However, according to this technique, since the accuracy of the magnetic pattern depends upon the mask patterning accuracy to some extent, for the effective use of this technique, there is a need to conduct minute processing on a mask with high accuracy. If the magnetic pattern is made more minute up to 1 to 2 μm or formed in terms of submicron, because of the limit to the processing accuracy, difficulty can be encountered in patterning a mask with high precision, and the alignment precision between the mask and the medium can deteriorate so that a magnetic pattern cannot be formed with sufficiently high accuracy.